1. Field of the Invention
This invention relates to an ultrasonic system including a flat annular transducer array constructed of a plurality of concentric rings in a common plane, in which the rings are connected to a transmitter and a receiver through a delay network which serves to delay transmitted and received electrical energy transmitted to or received by the rings of the transducer by progressively different time delays and, more particularly, to an ultrasonic system optimizing the transducer characteristics and improving the lateral resolution in the focal region of the transducer.
2. Description of the Prior Art
The lateral resolution of acoustical images depends primarily on the ultrasonic beam width. A known method of reducing the width of the ultrasound beam by electronic focusing is described in U.S. Pat. No. 4,169,385 issued in the names of Seeley C. Kellogg, Philip J. Peluso and Richard B. Bernadi on Oct. 2, 1979. It discloses an ultrasonic imaging system including an annular transducer array in which the elements of the transducer are energized at progressively different times to generate the desired shape of the beam. For example, as illustrated in FIG. 1, to focus at the focal length f, on the axis, the time delay Tp at any point P being distant x from the axis 1 of the array is given by: EQU Tp=f/c[{1+(r.sub.n /f).sup.2 }.sup.1/2 -{1+(x/f).sup.2 }.sup.1/2 ]
where r.sub.n is the outer radii of the outmost n-th ring of the array, and c is the velocity of the ultrasonic wave.
Equation (1) shows that the time delay varies continuously as a function of the distance x between any point P and the axis 1. In practice, the annular transducer array includes a certain finite number of discrete ring elements, and all the elements have equal widths, so that the time delay introduced by the delay lines varies inversely as a discontinuous function of off axis distance and still operates to increase sidelobe amplitudes.
Attempts to make small the path difference between adjacent rings by reducing the widths of rings and increasing the number of rings have resulted in excessive complexity and cost for a commercially acceptable system.